Synchronized communications and navigation system



June 11, 1968 w, GRAHAM ET AL 3,388,393

SYNCHRONIZED COMMUNICATIONS AND NAVIGATION SYSTEMI Filed Jan. 10, 196716 Sheets-Sheet 1 FIG. I

SYSTEM TIMING FOR SYNCHRONIZATION STAR; PULSE n l I; I I POSITION NO. I2 3 4 5 6 v, zoozoI Ff 5481 .Li. I gqg I 54? 1 TIME, sscouos 548548 548m Ix Rx STATION x I I a (TRANSMIT) I y STATION Y "I b (TRANSMITI 1y RSTATION x I c (RECEIVE) Ix x STATION Y "I d (RECEIVE) STATION x loMEASURES 20 e STATION Y MEASURES 2o f to t 2 SYNCHRONIZATION BETWEEN TWOSTATIONS (IN SYNC. CONDITION) INVENTORS WALTON GRAHAM STEPHEN PORTONATTORNEYS June 11, 1968 Filed Jan. 10, 1967 FIG. 3

W. GRAHAM ET SYNCHRONIZED COMMUNICATIONS AND NAVIGATION SYSTEMSheets-Sheet 2 I R STATION x I II o (TRANSMIT) y y STATION Y I II b(TRANSMITI y y STATION x I c (RECEIVE) Ix X STATION Y I I d (RECEIVE)STATION x B MEASURES e 7 STATION Y 9o f MEASURES to t Z| t At 2 62+! TOINTERROGATION g Jg'Ia g INTERROGATIQN PHASE PULSE OSCILLATOR SHIFTERGENERATOR AND TRANSMHTER m ADVANCE REPLY (RI PHASE II 3 RPY= RPY FIG 4 2I LO PA Z ERPY EINT INTERRO- B) GATION (I) PL'JbllSE EINT KTINTINVENTOR.

WALTON GRAHAM STEPHEN PORTON ATTORNEYS June 1968 w. GRAHAM ET AL3,388,393

SYNCHRONIZED COMMUNICATIONS AND NAVIGATION SYSTEM Filed Jan. 10, 1967 l6Sheets-Sheet 3 S=NUMBER OF SUCCESSFUL COMPARISONS TO REACHSYNCHRONIZATION I000 N y C I 1 E Mf a E X/ R O Q g T A I I 0 4 N s 2 I 24 8 I0 I00 IOOO I0,000 IO0,000

5 STATION Y STATION Z SSTATION X Fl 6.6m

FIRST INTERVAL SECOND INTERVAL x-| g x-z STATION X 0 TRANSMITS B I 1 x4J Lx-2 STATION Y b RECEIVES B,, STATION Y [I c TRANSMITS [E r4 neSTATION 2 d RECEIVES 8 STATION 2 II v e TRANSMITS COARSE SYNCHRONIZATION6B usme START (B) PULSES 'INVENTORS ATTORNEYS June 11, 1968 w GRAHAM ETAL 3,388,393

SYNCHRONIZED COMMUNICATIONS AND NAVIGATION SYSTEM Filed Jan. 10, 1967 16Sheets-Sheet 4 TR RECEIVING AT PRTT' 50s 50s FROM PULSE CODER (FIG. 23)

VARACTOR SWITCH SMALL DELAY MODULATOR LOCAL osc. fl

osc. A.V.C.

522 TIME sm $615; VARlABLE W GEN. 520 T 5l6 VIDEO AMP RECEIVERTRANSMITTER i To VIDEO DECODER (FIG. 23)

INVENTORS WALTON GRAHAM STEPHEN PORTON June 11, 1968 W. GRAHAM ET ALSYNCHRONIZED COMMUNICATIONS AND NAVIGATION SYSTEM Filed Jan. 10, 1967FIG. 8

l6 Sheets-Sheet 5 SYSTEM TIMING DURING DATA TRANSMISSIONPERIOD BASE MC0300' BASE sTATIoN A sTATIoN 8- f I PosITIoN"2 #4 TIME 0 L.

44 sEcoNos RANGE o 425 850 MILES R TRANSMITTER SWITCH REcEIvER I40 I34I50 I I I %AJ IA PULSE SYSTEM GENERAToRs MING DECODERS I36 I30 I32 GATESYNCHR NIZED SYNCHRONIZING OSCILLATOR CIRCUITS s PULSE II a PULSEPOSITIONS I I I I I I I I I b PHANTASTRON II c OUTPUT POSITION PULSE I dA 0R G PULSE I e GATE OUTPUT II f J INVENTORS WALTON GRAHAM STEPHENPORTON ATTGRNEY June-11, 1968 w. GRAHAM ET AL SYNCHRONIZEDCOMMUNICATIONS AND NAVIGATION SYSTEM 16 Sheets-Sheet 8 Filed Jan. 10,1967 @9 mo m "65556 mo wmii 33:2 29:81

mml

mob/122.532

INVENTORS WALTON GRAHAM BY STEPHEN PORTON fla/z ZS Zflqll 205.3% oznomowml 7 mg hom o u wmidm m AT TO RN EYS June 11, 1968 SYNCHRONIZEDCOMMUNICATIONS AND NAVIGATION SYSTEM w. GRAHAM ET Filed Jan. 10, 1967 16Sheets-Sheet 9 SYNCHRONIZED POSITION FOR RECEIPT OF OTHER OWN I PULSERECEIVED OWN |NTERR REPLY INTERR. POSITION PULSE POs TION A ERROR T E IT l POsmoN POSITION ECEIVED 201.. OTHER 202........ t

INTERR. PULSE suBTRACT C OJN$ COUNT SUBTRACT COUNT (FREQUENCY OF TIMINGPULSES DOUBLED) COUNT 2 IN SYNC. COUNTER t -r- 2 CORRECT PULSE k 3 OwNINTERR. POSITION CORRECTED POSITIONS SYNCHRONIZATION LEAD CASE INVENTORSWALTON GRAHAM BY STEPHEN PORTON ATTO RN EYS June 11, 1968 Filed Jan. 10,1967 SYNCHRONIZED POSITION FOR RECEIPT OF OTHER W. GRAHAM ET AL 16Sheets-Sheet 10 FIG. 135

I PULSE OWN RECEIVED OWN INTERR. REPLY INTERR. PO3|T|ON PULSE POSITION TJ T F 5 l s T Ex lNTERR t PULSE SUBTRACT INCREASB COUNT COUNT ZERO COUNTIN CROSSING 2 SYNC.

/-ADD COUNT Z Q (FREQUENCY OF mums PULSES DOUBLED) CORRECT PULSE 3 t-DOUBLE FREQ. TIMING PULSES ,3 EE% To COUNTER 220 A CORRECTED 1 4POSITIONS SYNCHRONIZATION LAG CASE INVENTORS WALTON GRAHAM STEPHENPORTON ATTORNEYS June 11, 1968 w. GRAHAM ET AL 3,388,393

SYNCHRONIZED COMMUNICATIONS AND NAVIGATION SYSTEM Filed Jan. 10, 1967 16Sheets-Sheet l4 I289 A/C POSITION GATE PULSE A0 BASE STATION A G TEPOSITION PULSE 0 29I I 292 I I mrolsvnc.

INFO- SYNC. INVERTER COUNTER 279 DELAY L I 7 POSITION I I PULSES I FROMI COUNTER ZZOIFIG I4) 1 l GATE DELAY [287 GATE B 9 START POSITION PULSEPULSE POSITION SELECTOR (To 2?) FOR B,A ANDG PULSES INVENTORS WALTONGRAHAM STEPHEN PORTON.

' ATTORNEYS June 11, 1968 w, GRAHAM ET AL 3,388,393

SYNCHRONIZED COMMUNICATIONS AND NAVIGATION SYSTEM Filed Jan. 10, 1967 16Sheets-Sheet l5 "owN" INTERROGATION POSITION PULSE SELECTOR IlowN llINTERROGATION POSITION osggr guu g PULSES FROM 3 GATE GATE COUNTER 220 nI I FL FI IINFOI SYNC 307 FROM 292 i BLOCKING RA 00 (HQ OSCILLATORPERIQD 308 8 (FROM 486 OF FIG.23)B PULSE T0 THRESHOLD /3|o COUNTERANALOG Bo (REsET) NOISE GENERATOR 305 FROM 287OF 320 FIG. I?) RECEIVE 0328 DECODED START PULSES AND AND BISTABLE CL SE GATE GATE FLIP-FLOP GATE2m 7 A AND REsET COUNTER 22o [mrojsmcl (FIG. l4)

FROM 292 (FIG. I?) 32 T REsET T 322 324 DIFF. BISTABLE AND d/m FLIP-FLOPGATE START PULSE coARsE SYNCHRONIZATION I coRREcT PULSE FROM 209(FIG.l4)

400 CPS 35 GEA RTRAIN ZERO CRCE)SS 0 (FROM FLIP-FLOP 225 FIG [4)FREQUENCY CORRECTION V?LTAGE MASTER TIMING OSCILLATOR INVENTORSFREQUENCY CONTROL T N GRAHAM STEPHEN PORTON BY fi -29 ATTORNEYS UnitedStates Patent 3,388,393 SYNCHRONIZED COMMUNICATIONS AND NAVIGATIONSYSTEM Walton Graham, Roslyn, and Stephen Porton, Wyandanch, N.Y.,assignors to Control Data Corporation, Minneapolis, Minn., a corporationof Minnesota Continuation-impart of application Ser. No. 565,779,

July 18, 1966. This application Jan. 10, E67, Ser.

6 Claims. (Cl. 3437.5)

ABSTRACT OF THE DISCLOSURE This invention relates to a system forproviding range, bearing and/or altitude information for a plurality ofstations with respect to each other in order to provide for navigation,collision avoidance and air trafiic control capabilities whereinsynchronized reference signals are produced at all stations by asemi-random comparison between stations and in which adjustment ofreference signal clocks is by advance only.

This application is a continuation-in-part of copending application Ser.No. 565,779, filed July 18, 1966, for Synchronized Communications Systemin the name of Walton Graham which is in turn a continuaton-in-part ofapplication Ser. No. 328,655, filed Dec. 6, 1963, now Patent No.3,262,111 for Synchronized Communications System in the name of WaltonGraham.

I. INTRODUCTION In applicants prior copending applications Ser. No.35,659, filed June 13, 1960, entitled A Compatible AirborneNavigation-Air Trafi'ic Control and Collision Avoidance System now US.Patent No. 3,183,504, and Ser. No. 42,886, filed July 14, 1960, nowPatent No. 3,255,900 and having the same title, both of which areassigned to the assignee of this application, systems were disclosedwhich provided navigation, collision avoidance and air trafiic controlcapabilities for a plurality of stations. The systems previouslydisclosed utilize a master station which transmits synchronizing signalsto a number of fixed base stations. The fixed base stations, which areat a known distance from the master station and therefore have a knowntime delay between the transmission and reception of the synchronizingsignals, use these synchronizing signals to become synchronized with themaster station, thereby providing a network of synchro U nized basestations.

Each of the base stations in those systems transmits reference pulses ata fixed rate and on a different carrier frequency. These referencepulses are received by a number of movable stations, such as aircraft.The movable stations transmit interrogation pulses to which the basestations respond by transmitting reply pulses. Enough different carrierfrequencies are used to permit unambiguous interrogation of and reply bya particular base station.

Since each movable station can determine its range to a base station,thereby knowing the time of propagation of the base station referencepulses, it is possible to synchronize the interrogation pulses of themovable stations with the base station reference pulses. Therefore,since every movable station is synchronized with one or another basestation, all of the base stations being 'syn chronized with each other,all of the movable stations are in synchronism. Consequently, everymovable station is capable of measuring the range to every other movablestation or base station by observing the time of arrival of pulsesfromthose stations. By restricting the time of 3,388,393 Patented June 11,1968 transmission of various ones of the pulses from the moving stationsto certain transmission positions, it is possible to provide additionalinformation concerning the movable station, such as its altitude. Also,it is possible to make measureemnts on various received pulses in orderto determine the bearing of one station from another. Therefore, theseprevious systems provide a complete arrangement having navigation,collision avoidance and air traffic control capabilities.

While the aforesaid systems provide a complete working arrangement forthe desired operations, several disadvantages are present. First of all,the presence of a number of base stations is required and these basestations must be synchronized with each other at the added cost ofproviding an auxiliary system for synchronization. Additionally, theoperation of the various base stat ons on different frequencies requiresthe use of a substantial radio frequency bandwidth out of the alreadycrowded frequency spectrum and also requires transmitters and receiversfor the movable stations which must be both stable in frequency andtunable over the frequency band. Since the receivers for the movablestations must have a bandwith which is adequate to receive interrogationpulses transmitted at many possible frequencies in a wide frequencyband, the range of operation of the systems is limited by the design ofthese wide band receivers.

Application Ser. No. 328,655, now Patent No. 3,262,- 111, is directed toa system for navigation, air traffic control and collision avoidancewhich eliminates many of the aforesaid disadvantages and also introducesunique operating advantages. In that system, no master station is neededand there are no intermediate base stations which must be synchronizedwith each other by some auxiliary means. Instead, every station acts tosynchronize with every other station, whether the station is fixed ormovable. Also, all transmissions and receptions from movable or fixedstations occur at the same frequency, thereby considerably reducing thecost of each stations transmitter and receiver and at the same timeallowing for greater receiver sensitivity and operation of thetransmitters at relatively low powers.

In the preferred form of synchronizing system disclosed in Patent No.3,262,111, each of two comparing stations made one-half of the clockadjustment necessary to bring them into synchronism.

The preferred embodiment of this application provides great advantagesover the previous systems in terms of simplicity, reliability and speedof synchronization, largely by virtue of a synchronization procedure inwhich substantially all the indicated correction between two comparingstations is made by the station with the lagging time rerefence clock.This is referred to as advance-only synchronization.

An object of this invention is to provide a system for synchronizing thetransmissions of a plurality of stations by using only the transmissionsthemselves in which synchronization is simplified and expedited byarranging for advance-only corrections to attain synchronism.

A further object of the invention is to provide a system forsynchronizing the transmissions of various stations, both fixed andmobile, and using these transmissions to provide range and altitudeinformation of one station with respect to another.

Other objects and advantages of the present invention will become moreapparent upon reference to the following specification and annexeddrawings, in which:

FIGURE 1 is a timing diagram showing various transmissions from base andmovable stations;

FIGURE 2 is a timing diagram showing the transmitted and received pulsesof two stations in the synchronized condition;

reassess FIGURE 3 is a timing diagram showing the transmitted andreceived pulses for two stations in the unsynchronized (out'of-sync)condition;

FIGURE 4 is a schematic block diagram of one type of circuit forcorrecting the phase of an oscillator to obtain synchronization;

FIGURE 5 is a graph showing the number of comparisons between stationsrequired to reach synchronization;

FIGURE 6A is a schematic diagram showing the locations of threehypothetical stations;

FIGURE 6B is a timing diagram showing the use of START (B) pulses toachieve coarse synchronization between the three stations;

FIGURE 7 is a block diagram of a receiver-transmitter for use with thesystem;

FIGURE 8 is a timing diagram illustrating certain operating principlesof the invention;

FIGURE 9 is a simplified block diagram of the system of the invention;

FIGURE 10 is a detailed block diagram of typical components which may beused in the system of the present invention;

FIGURES 11A, 11B and 11C are schematic block diagrams of portions of thesystem of the present invention;

FIGURE 12A is a schematic block diagram of the altitude gating circuit;

FIGURE 12B is a timing diagram showing the operation of the altitudegating circuits;

FIGURES 13A and 13B are timing diagrams showing correction forsynchronization using a pulse counting technique;

FIGURE 14 is a block diagram of the equiment at a station for producingsynchronization using pulse counting;

FIGURES 15A and 15C are block diagrams of different types of reversiblecounters while FIGURE 15B illustrates binary counting techniques;

FIGURE 16 is a block diagram of a circuit used to prevent production ofinformation pulses when the station is unsynchronized;

FIGURE 17 is a block diagram of a circuit for selecting the positionpulses for the B A and G, pulses;

FIGURE 18 is a block diagram of a circuit for selection of positionpulses for the I pulses;

FIGURE 19 is a block diagram of a circuit for producing coarsesynchronization of the start pulses;

FIGURE 20 is a frequency control circuit for the master timingoscillator of FIGURE 14;

FIGURE 21 is a block diagram of a circuit for selecting position pulsesto produce the A pulses; and

FIGURES 22 and 23 are block diagrams of a pulse coder and decoder,respectively.

II. SYSTEM SIGNAL TRANSMISSIONS In order to explain the operation of thesystem of the present invention, the following symbols are adopted forthe various transmissions:

R-reply pulses Iinterrogati0n pulses B--start pulses G-ground stationinformation pulses Aairborne station information pulses 0subscriptdesignating a signal transmitted by own station Throughout thedescription, the term base station is used to mean a station which isfixed relative to the movable stations. The base station, for example,may be a fixed ground station or a relatively stationary beacon stationoperating on the water. The term movable station is used to define thosestations which move relative to the base stations and/or to each other.These may be, for example, aircraft, helicopters, or other types ofstations moving in the air, on the ground, or on the Water. It should berealized that other stations will fall within the definition of base ormovable in the manner as defined herein.

In order to explain the operation of the system, reference is made toFIGURE 1 which shows some of the signals that are transmitted duringeach operating interval which, for the purposes of explanation isassumed to be one second.

The one second interval is divided up illustratively into 548transmission positions occurring every second. At t equal 0 second andat transmission position #1 the transmission interval begins with aSTART pulse B which is transmitted by each operating station. Positions#2 to #200 are data transmission positions with the even numberedpositions, #2, #4 #200 assigned to particular base stations fortransmission of base station information pulses G and the odd numberedpositions starting at #3 and up to #199 assigned to the movablestations. In the illustrative system application being described, theodd numbered positions are for transmission of airborne stationinformation pulses A in accordance with the altitude layer within whichthe respective airborne stations are located.

For example, with respect to the base stations, base station X isassigned to transmit an information pulse G at position #2, base stationY to transmit an information pulse G at position #4 and base station ZZto transmit information pulse G at position #200. Since a particularbase station transmits an. information pulse only at its assignedtransmission position with respect to start pulse B, a base station maytherefore be identified by its transmission position. It should berealized that the same transmission position can be used for two basestations provided the two base stations are sufficiently far enoughapart, so that the transmissions from one base station cannot bereceived by a movable station operating within range of the other basestation.

In the illustrative embodiment of the invention being described, the oddnumbered positions #3 #199, for transmission of aircraft informationpulses, correspond to a plurality of successive altitude layers of 300feet. Therefore, those aircraft in the altitude layer from 0-300 feettransmit information pulses A at position #3, those aircraft in thealtitude layer from 300-600 feet transmit pulses A in position #5, andso forth. It should be realized that a plurality of aircraft may belocated in a particular altitude layer and each of these aircraft willtransmit its respective altitude information pulse only at the properposition. The height of the altitude layers can be established inaccordance with the complete system requirements, which includes theoperating altitudes of the various aircraft. It should also be realizedthat each altitude layer does not have to be the same height but, forexample, the height of the respective altitude layers can be increasedwith increasing altitude in order to take into consideration the factthat the accuracy of aircraft altimeters decreases with increasingaltitude. In this case, therefore, the upper altitude layers would be ofgreater height than the lower altitude layers. For example, the lowerlayers would be 300 feet each and the upper layers 1000 to 1500 feeteach.

Transmission positions #201 to #548 of FIGURE 1 are used by both thebase stations and the movable stations to transmit interrogation pulsesI for the purpose of obtaining and maintaining synchronization among allof the stations. The achievement of synchronization is necessary inorder that the various stations may be able to transmit information atthe proper positions and identify all the positions to obtaininformation from the other stations. According to the principles of theinvention, no station, fixed or movable, transmits information pulses Gor A in positions #2 to #200 unless it is synchronized. Thedetermination of synchronization condition is automatically recognizedby each station by the production of zero or minimal error signals inthe stations error determining circuits and synchronization is achievedin a manner to be described.

Once the various stations are synchronized, they are able to determinethe range to and altitude of the other stations within theirtransmission and reception range. For example, upon obtainingsynchronization each movable station knows the transmission position andtime of an information pulse G from a particular ground station and themeasurement of the time between the transmission position and thereception of the G information pulse gives the range from the movablestation to the ground station. A ground station is able to determine therange and altitude of aircraft information pulse A after the occurrenceof an odd numbered position and noting the transmission position number.In the same manner, each aircraft can determine the ran e to andaltitude of every other aircraft.

While the system is described as using 548 transmission positions, ofwhich 199 are used for information pulses and 348 for synchronizationpurposes, it should be realized that other transmission position rates,either higher or lower, may be used. Also, the rates of the informationand synchronization positions may be different, if desired. The choiceof the proper rates depends upon a number of factors, including stationdensity (number of stations operating in a given area), desired range ofoperation between stations, system power, etc.

To summarize the various pulses that are transmitted by the system, atthe start of each transmission interval, each station, fixed or movable,transmits a start pulse B at position #1; information pulses G and A aretransmitted by the base and movable stations respectively at therespective even and odd positions #2 to #200; and interrogation pulses Iare transmitted by bot-h the fixed and movable stations from positions#201 to #548 for synchronization purposes.

During the time allotted for synchronization (positions #201 to #548),in addition to the above pulses each station transmits a reply pulse R(not shown in FIG. 1) under certain conditions. The reply pulse istransmitted by a station in the time interval between the transmissionposition at which the station transmitted its own interrogation pulseand the next transmission position. A reply pulse is also transmitted bya station only after the first interrogation pulse from another stationis received and the reply pulse is used by the other station to achievesynchronization.

Thus, during the time allotted for synchronization, each stationtransmits its own I pulses and receives I pulses from other stations.Each station also transmits R pulses in reply to certain received Ipulses and receives R pulses transmitted from other stations in responseto the first mentioned stations own transmitted I pulses. As isdescribed below, each station uses its own I pulses and the R pulsesreceived from other stations in response to its own I pulses to achievesynchronization.

III. SYNCHRONIZATION OF TWO STATIONS To explain how the many stationsoperating in an overall field (i.e., all those stations, fixed ormovable, operating together to achieve common synchronization) ofstations are synchronized, reference is made to FIGURE 2 which shows thepulse transmissions which occur when two stations in the field aresynchronized. The term synchronized may be explained as follows.Starting with the condition that a pulse is transmitted by each stationat the instant that an oscillator at the station which controls thepulse producing means has a certain phase angle, for example, at thetime of a positive zero crossing of a sine wave, two such stations andtheir respective oscillators are considered to be synchronized when thepulses transmitted by each station at a particular instant are observedsimultaneously at a point midway between the two stations. The conditionof synchronization to be described holds true for two movable stations,a fixed and a movable station, or two fixed stations. It should berecalled that the stations operate to achieve synchronization onlyduring positions #200 to #548. However, once the synchronized conditionis obtained by a station, it is held during the other positions #1 to#199 by various circuits at the station.

In order to obtain synchronization between the stations duringtransmission positions #200 to #548, each station transmits two kinds ofpulses which are respectively interrogation pulses I and reply pulses R.Each station transmits interrogation pulses I in some of the positionsafter #200 and reply pulses R in response to the reception of the firstinterrogation pulse I from another (second) station received aftertransmission of the first stations own I pulse. The first I pulsereceived would be from the closest station in the field of stationswhich happened to transmit at the same position.

In order to achieve the synchronized condition each station attempts tosynchronize the transmission of its own interrogation pulse I with theinterrogation pulse transmitted by the other station. Each station doesthis by determining its range to the other station. This is accomplishedby having each station measure the elapsed time between transmission ofthe stations own interrogation pulse and reception of the reply pulsestransmitted by the second station in response to this interrogationpulse. Since the round trip time between the transmission of theinterrogation pulse by a station and the reception of the reply pulsetransmitted by the other station in reply to this interogation pulsemultiplied by the velocity of propagation of the signal is equal totwice the range between the two stations, the actual range between thetwo stations can be determined by dividing the overall round trip rangeby two.

By measuring the range between stations, each station is also able todetermine the actual transmission time of the interrogation pulse Ireceived from the other station. This is done by determining the time ofreceipt of the other stations interrogation pulse with respect to thefirst stations own interrogation pulse transmission time and the receiptof the reply pulse from the other station. If a stations owninterrogation pulse was not transmitted in synchronism with theinterrogation pulse of the other station, a correction is made to thestations master oscillator so that the system pulse transmission time iscorrected on subsequent transmissions.

FIGURE 2 shows the synchronized condition for two Stations X and Y.Lines a and b show the pulses transmitted by Stations X and Yrespectively while lines 0 and d show the pulses respectively receivedby Stations X and Y. At time t=t in the in-syn-c (synchronized)condition, both stations X and Y transmit the respective interrogationpulses I and 1,, (lines a and b). This transmission would occur at anyof positions #201 to #548. Pulses I and I are received at the respectiveStations X and Y at time t=t The time between transmission of I (or 1,)and the reception of I (or I is called T and may be measured as avoltage E In response to the reception of the interrogation pulses attime t t Station X transmits reply pulse R and Station Y transmits replypulse R (lines a and b). Reply pulses R and R are received at therespective Station X and Y at time t=t (lines o and d). The time betweentransmission of I (or l and reception of R (or R is called T and may bemeasured as a voltage E or as a number of pulses counted from a clock.

For explanatory purposes, line e shows that Station X measures 10 unitsof time (T and/or distance between times t t and 1:13, i.e. between thetransmission of its interrogation pulse I and the reception ofinterrogation pulse I from Station Y. Station X measures 20 units oftime (T or distance between the transmission of its own interrogationpulse I and the reception of reply pulse R Since the interrogationpulses I and I are initially synchronized, Station Y also measures thesame respective number of units of time or distance (10 and 20) betweenthe transmission of its own interrogation pulse I and the reception ofinterrogation pulse I and between transmission of I and the reception ofreply pulse R When synchronization exists there is a l ratio betweenTRPY INT

ERPY

EINT

These time and/or distance measurements may be made in analog fashion bystoring a voltage proportional to time on a capacitor or digitally, bycounting pulses generated by a high frequency clock. Both of thesetechniques are well known in the art.

It should be noted that when the stations are synchronized the rangefrom Station X to Y is determined at Station X by measuring the elapsedtime between transmission of pulse I and reception of pulse R Station 'Ydetermines the range to X by measuring the elapsed time betweentransmission of T and reception of R,;. The range between stations isequal to one-half the time measured multiplied by the velocity ofpropagation. Also, the range between the two stations is equal to thetime of transmission of I (or l and the time of reception of I (or Imultiplied by the velocity of propagation.

As can be seen, if the interrogation pulses I and 1,, are intiallysynchronized, each station receives the interrogation pulse transmittedby the other station midway in time between transmission of its owninterrogation pulse and reception of the reply pulse transmitted by theother station. This follows from the fact that the elapsed time betweentransmission of an interrogation pulse by one station and reception ofthe interrogation pulse which was transmitted at the same time by theother station equals the distance between the stations divided by thevelocity of propagation. The elapsed time between transmission of aninterrogation pulse by one station and reception of a reply pulsetransmitted by the other station upon reception of this interrogationpulse is equal to the twoway (round trip) distance between stationsdivided by the velocity of propagation.

It should be noted that when there is a relative velocity between thetwo stations a small error in measurement will occur. This error isproportional to the range between the stations and their relativevelocity. In a typical case of a 200 mile range between the stations and1000 feet per second relative velocity, this error is only in the orderof 10 seconds, which is negligible with respect to the overall accuracyof the system.

When the interrogation pulses from the two stations are not initiallysynchronized, one station will conclude, after making measurements ofthe elapsed time between transmission of its interrogation pulse andreceipt of the interrogation and the reply pulses from the otherstation, that its own interrogation pulse was late (or early) withrespect to that of the other station. The other station will concludethat its interrogation pulse was early (or late) with respect to thefirst station. Therefore, at both stations the 2:1 ratio of time orvoltage measurements, in which the larger quantity T or E of the ratiois produced by measuring the interval from the transmission of an 1pulse to the reception of an R pulse and the smaller quantity T or E bymeasuring the interval between the transmission of an I pulse and thereception of another I pulse, will not be present.

The unsynchronized condition between two stations is shown in FIGURE 3,wherein Station Y transmits its interrogation pulse I at a time t=t +Atwhich is A: late with respect to the transmission of interrogation pulseI at time t=t It can be seen (line 0) that Station X receives pulse I ata time which is more than halfway between the time of transmission ofits own interrogaknown in the art. One such tion pulse I and the time ofreception of the reply pulse R which was transmitted by Station Y inresponse to interrogation pulse I Similarly, it can be seen (line d)that Station Y receives interrogation pulse I at a time less thanhalfway between the time of transmission of its own interrogation pulseI and the time of reception of the reply pulse R which was transmittedby Station X in response to interrogation pulse I The magnitude of thediscrepancy in the ratio of the two times measured by Stations X and Yfrom the ratio of 2:1 for the synchronized condition is proportional tothe synchronization error. This is quantatively shown in FIGURE 3, whereit can be seen that Station X measures 13 units from time t=t to thereception of pulse I at time t:t +At and 20 units from time t=t to thereception of reply pulse R at time t=t Station Y measures 7 units fromthe time of transmission of pulse 1,, at time t=t +At to the receptionof pulse I at time t=t and 20 units from the time r=t +ot to thereception of reply pulse R at time r=r +At. This gives the respectivetime or voltage ratios of 30:13 and 20:7 for Stations X and Y and thestations are therefore not synchronized. If the error in transmissionwas reversed, i.e., Station X transmitted later than Station Y, then themeasured quantities and ratios would also be reversed. In either case,the error produced by the deviation of the measurement ratio from thesynchronized ratio of 2:1 is used to produce synchronization.

The operation of present system is such that at each station the timebetween transmission of its own interrogation pulse I and reception ofthe reply pulse R, generated by another station upon the reception ofthe I pulse, is independent of synchronization and depends only on thedistance between the two stations as in a conventional beacon system.The distance is measured by a time quantity. On the other hand, the timeof reception of the interrogation pulse transmitted by the other stationdepends on synchronization and the distance between the two stations.Thus, at each station if the interrogation pulse received from the otherstation lies in time less than or greater than halfway between the timeof transmission of an interrogation pulse and the reception of the replypulse transmitted by the other station in response to this interrogationpulse, it is determined that the two stations are out of synchronism.The amount of error greater than or less than the halfway distance intime is used to correct the oscillators of the late station so that thestations are driven to the synchronized condition.

The time-synchronization error can be measured in any of a number ofways. For example, capacitor and switching circuits can be used in whichthe stations own interrogation pulse starts two capacitors chargingtoward a predetermined voltage level established by a voltage source byactivating an electronic switch which is connected between eachcapacitor and the voltage source. The charge up of one of the capacitorsis terminated by the interrogation pulse received from the other stationto measure T and product E by having this pulse operate the switch todisconnect the circuit from the voltage source. The charge up of theother capacitor is terminated by the reply pulse received in response tothe stations own interrogation pulse to measure T and produce E Thevarious pulses are sorted out and applied to the proper circuits bypulse decoders in the stations receiver. Also the capacitor whichmeasures the time T between the interrogation and the reply pulsescharges at one-half the rate of the one that measures the time T betweenthe transmitted and received interrogation pulses. Therefore, when thetwo stations are synchronized, the voltages E and E on the twocapacitors will be equal. If the stations are not synchromzed then anerror voltage is produced which is used to control the phase of aninterrogation pulse oscillator at one station. it should be realized, ofcourse, that any suitable time measuring circuits may be used, as iswell circuit would be of the

